The giant magneto-resistive effect (GMR effect) may be utilized, in the form of so-called spin-valve structures (“spin-valves”) for angular-position sensing. This is described, for example, in PCT International Publication No. WO 00/79298 or in European Published Patent Application No. 0 905 523 A2.
GMR spin-valves are made up in essence of two ferromagnetic thin films having a resulting magnetization m1 or m2, which are separated from each other by a nonmagnetic thin film lying in between. The electrical resistance R(α) of such a layer system then shows a cosine-type function of the angle α between the direction of magnetization m1 and the direction of magnetization m2 of the form:R(α)= R−Q5·ΔRGMR·cos(α)
In this context, the maximum relative resistance change ΔRGMR/ R designates the GMR effect, and typically amounts to 5% to 10%. GMR spin-valve layer systems, by the way, are usually deposited with the aid of cathode sputtering of the respective materials, and then structured using customary photolithography methods and etching techniques.
What is essential for the intended spin-valve function is a rigid, at least approximately not changeable direction of magnetization m1 of the first ferromagnetic layer, of the so-called reference layer (RL), because of a magnetic field, acting from outside on the layer system, that is to be detected particularly with regard to its direction and/or strength, and a direction of magnetization, m2, of the second ferromagnetic layer, of the so-called free layer (FL) or detection layer, that orients itself slightly, at least approximately parallel to the outer magnetic field. In order to achieve both of these, on the one hand, the two ferromagnetic layers are magnetically decoupled by a sufficient thickness of the nonmagnetic intermediate layer, of the so-called nonmagnetic layer (NML), of typically a few nanometers, and the magnetization of the reference layer (RL) is fixed or “pinned”, for instance, by an additional, directly adjacent antiferromagnetic layer, a so-called natural antiferromagnet (AF), and by its mutual magnetic coupling by exchange interaction.
This is shown schematically in FIG. 1a, where the GMR layer system or GMR sensor element is under the influence of a magnetic field of a magnetic transducer. One may achieve a further improved stabilization of the reference magnetization by adding an additional so-called synthetic or “artificial” antiferromagnet (SAF). This SAF, corresponding to FIG. 1b, is made up of two ferromagnetic layers that is strongly antiferromagnetically coupled via a nonmagnetic intermediate layer. The ferromagnetic layer of these two, which lies directly next to or on the natural antiferromagnet AF, is designated as the pinned layer (PL), since its magnetization MR is fixed or “pinned” as a result of the coupling with the natural antiferromagnet (AF). The second ferromagnetic layer of the SAF, whose magnetization MR is oriented opposite to that of the pinned layer (PL) as a result of the antiferromagnetic coupling, is used as reference layer (RL) for the abovementioned GMR spin-valve layer system.
In order to extract the angle-dependent useful signal, in a GMR sensor element according to the related art, four spin-valve resistance elements are connected together to form a Wheatstone's bridge circuit (Wheatstone's full bridge), such as by using an aluminum thin film track conductor. The maximum signal amplitude is obtained by, as in FIG. 2, oppositely oriented reference magnetizations MR of the bridge resistors within the half bridges and similarly oriented reference magnetizations MR of the resistors lying diagonally in the full bridge.
As a rule, a GMR angle sensor also has a second full bridge of GMR resistors, whose reference directions, as shown in FIG. 2, are rotated by 90° to the ones of the first bridge. Signal Usin made available by the second full bridge is thereby phase-shifted by 90° with respect to the signal of the first full bridge Ucos.
By arctangent formation or corresponding algorithms (such as the CORDIC algorithm) one then determines, from the two cosine-shaped or sine-shaped bridge signals Usin, Ucos, the angle α, that is single-valued over a full 360° revolution, to the direction of an outer magnetic field B.
The different reference magnetization directions according to FIG. 2 are, for instance, implemented in that the individual GMR bridge resistors are heated locally to a temperature T above the blocking temperature (Néel temperature) of the antiferromagnetic layer (AF), but below the Curie temperature of ferromagnetic layers (PL, RL) as in FIG. 1a or 1b, so that the antiferromagnetic spin order in the antiferromagnetic layer is canceled, and thereafter they are cooled in an outer magnetic field of a suitable field direction. In the renewed formation, taking place in this context, of the antiferromagnetic order, the spin configuration resulting from the exchange interaction at the interface of antiferromagnetic layer (AF) and adjacent ferromagnetic layer (PL) is frozen. As a result, the direction of magnetization of the adjacent ferromagnetic layer (pinned layer PL) is fixed. The local heating of the GMR bridge resistors may take place, for example, with the aid of a brief laser pulse or current pulse. The current pulse may be driven, in this context, directly by the GMR conductor structure or/and an additional heating conductor.
In the case of known GMR angle sensors, reference magnetization MR of the individual bridge resistors is selected to be either parallel or perpendicular to the direction of the strip-shaped structured GMR resistor elements. This is used to hold the influence of the shape anisotropy to a low value. Furthermore, the strip-shaped structured GMR resistor elements are preferably aligned in parallel within a full bridge according to FIG. 2. This is used to suppress a signal contribution because of a superimposed anisotropic magnetoresistive effect (AMR effect). The AMR signal contribution is based, in this context, on a function of the electric resistance of the angle α between the current direction and the magnetization direction of the form:R(θ)= R+Q5·ΔRAMR·cos(2·θ)
If, on the other hand, the GMR resistors are implemented within a half bridge and having orthogonal alignment of their GMR strips, as is the case, for example, in FIG. 10 in PCT International Publication No. WO 00/79298, then the AMR signal contribution is even maximally favored. That acts in a worsening manner on the angular accuracy of the GMR angle sensor.